Rapid loading of steam turbines

ABSTRACT

A method for loading a steam turbine and transferring from full arc to partial arc admission during loading while maintaining a constant rate of heating, so as to eliminate the cooling and reheating which previously took place during a transfer at constant load.

United States Patent lnventors Eric Manuel Peterborough, Ontario, Canada; James H. Moore, Jr;, Scotia, N.Y. 822,933

May 8, 1969 May 4, 1971 General Electric Company July 16,1968

Canada App]. No. Filed Patented Assignee Priority RAPID LOADING 0F STEAM TURBINES 3 Claims, 5 Drawing Figs.

US. Cl 60/105, 60/73 [5]] Int. Cl Folk 13/02 [50] Field of Search 60/73, 105, 108

[56] References Cited UNITED STATES PATENTS 3,338,053 8/1967 Gorzegno 60/105 SLEAM FROM STEAM GENERATOR.

RATED 1000F-24OOPSIG.

BYPAss VALVE 15 FIRST STA GE HIGH PRESSURE SECTION.

COLD REHEAT REHEAT BOWL Y STEAM- APPX. 16 CONTROL 18 600 F. VALVES.

HOT REHEAT STEAM RATED 1000F Primary Examiner-Martin P. Schwadron Assistant Examiner-Allen M. Ostrager Attorneys-William C. Crutcher, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman ABSTRACT: A method for loading a steam turbine and trans ferring from full arc to partial arc admission during loading while maintaining a constant rate of heating, so as to eliminate the cooling and reheating which previously took place during a transfer at constant load.

MAIN STOP VALVE.

13 VALVE CHEST.

STEAM FLOW.

INTERCE T VALVE. P

STEA M FLOW.

FIRST LOW PRESSURE 2 SECTION.

STEAM TEMPERATURE AT ENTRANcE SHEET 1 OF 3 MA|N STOP VALVE.

BVALVE CHEST.

REHEAT STOP 17 VALVE.

RA ED 10oo F24OOPS|G.

BYPASS VALVE 15 FIRST STAGE -ARP CONTROL .VALVES.

HOT REHEAT STEAM RATED 1000F PATENTEU HAY 4|97| ST EAMFROM STEAM GENERATOR.

CLD REHEAT STEAM 6ooE T mm 3 I E n 2 R m 5 A U R G R an 0 Wm R m FX. EM.

5 A Am 9E6 2 WW HH.| SA

m SE53: zwEo wm SoEzou o: z wank/E525 9 5 9 636 2% z 9 5% 9 6 392a 0mm 2 mmnwmmmm TIME IN HOURS N ORSI ERIC MANUEL,

JAMES H. MOORE,JR.

SHEET 2 [1F 3 CHEST METAL DIFFERENTIAL TEMPERATURE. OUTSIDE METAL TO INSIDE METAL TEMPERATURES CONTROL FROM THERMOCOUPLE SIGNALS. STEAM'TO OUTER METAL DIFFERENTIAL TEMR T J ML ML 100- CHEST DIFF. TNNER TO OUTER METAL TEMP DIFF. TEMP.

CONTROL VALVE POSITION (PRIOR ART) O u 1 I l I /o TRANSFER TIME.

CHEST METAL DIFFERENTIAL TEMPERATURE. OUTSIDE METAL To sTEAM 0R INSIDE METAL TEMPERATURES.

CONTROL FROM CHEST PRESSURE.

CHEST DlFF.

TEMP. F.

SIGNALS.

CQNTROL VALVE P SITIO o N I s Q i l I T 2O 4O .60 INVENTORS:

loTRANSFER TIME. ER|C MANUEL,

FIG.4. .JAMEs H. MO0RE.JR.

IBIS-{1733 PATENTED'MAY 41971 SIITET 3 [IF 3 HOT START-UP RAPID LOAD PROFILE.

TRANSFER WHILE LOADING.

E m wzoEozoo 95E W 046. 4 50 PARTIAL ARC LOADING 20 TIME FROM SYNCHRONIZATION MINUTES.

FIG 5.

YINVENTORSZ ERIC MANUEL, JAMES H.,MOORE.JR.

RAPID LOADING OF STEAM TURBINES BACKGROUND OF THE INVENTION This invention relates to a method for decreasing the time required for loading steam turbines during startup thereof.

Bringing a large steam turbine up to full load operation, i.e., turbine startup, involves heating large masses of metal from an initial temperature, which can be measured, to an operating temperature, which is known, by the flow of hot steam on one side of the thick masses of metal. Heating metal in this way leads to temperature gradients and internal stresses of magnitudes directly related to the rate of heating, that is, the higher the heating rate, the greater the thermal stresses. Many factors affect the heating rate, the most important of which are steam temperatures and the loading of the turbine. These thermal stresses are repeated every time that the turbine is started, and after a number of starts metal fatigue sets in, resulting in surface cracks. The number of turbine startups possible for a given metal heating rate before cracks appear has become known from experience, and turbine operating procedure is now based on a specified number of startups. A common figure is an average life expenditure of 0.02 percent per startup, i.e., 5000 startups. This figure along with other life expenditures appear on charts wherein the amount of metal temperature change is plotted against rate of metal temperature change from data collected over the years.

In the past, electric utilities have generally used their newer large steam turbine generating units for carrying the base loads, and their older smaller units for supplying the recurring peak loads. Since the larger units operate for long periods of time without stopping, the time taken for starting and loading the turbine is relatively unimportant in the overall program; it is such a small factor in the economics of a run that it can be very long indeed. It is, however, very important that this time be long enough that the rate of turbine metal heating not exceed the specified rate. As a result, the operating procedure provided for a long interval of starting time, an interval that tended to be longer than necessary rather than too short. It is now proposed that large steam turbines also be used on standby for supplying electrical energy to the system during the recurring peak loads. As a result, there is now a real need for a reduction in the time taken for heating the turbine to the temperature where it is delivering its rated output. This must, of course, be done without exceeding the rate of turbine metal heating mentioned above. I

The object of this invention is to reduce the time required for bringing a steam turbine up to rated output of the generator.

DRAWING A turbine startup procedure according to the invention will now be described with reference to drawings, in which FIG. 1 and elementary diagram of a multistage steam turbine;

FIG. 2 is a graph illustrating the development of the rapid starting procedure for a hot startup;

FIG. 3 and 4 are graphs illustrating chest temperatures and pressures; and

FIG. 5 is a graph of a hot startup.

SUMMARY OF THE INVENTION Briefly stated, the invention comprises a method of loading a steam turbine with stop valve bypass and control valves, comprising the steps of l loading on the bypass at a specified rate of heating; 2 transferring to control valves while maintaining the rate of heating and 3 continuing to load at the same rate of heating.

DESCRIPTION FIG. 1 is a diagram of a large, high pressure, high temperature steam turbine having a high pressure section 10, an intermediate pressure section 11, and one or more low pressure sections 12. The high pressure section receives steam from the steam generator by way of a valve chest 13 which includes a main stop valve 14, at least one bypass valve 15, and a number of control valves 16. After the high pressure section has extracted considerable heat from the steam, it is returned to the boiler for reheating and then fed to the intennediatepressure section by way of control means which includes a reheat stop valve 17 and an intercept valve 18. After expending much of its energy in the intermediate pressure section, the steam then passes to the low pressure sections as illustrated. This is a known turbine installation which requires no further description for an understanding of the invention by those knowledgeable on the subject.

At the beginning of a turbine startup the main stop valve 14 is closed, all the control valves 16 are fully open, and the flow of steam is controlled by the bypass valve 15. An even flow of steam is therefore admitted to the full arc of the first nozzles, i.e., full arc control. Reheat stop valve 17 and intercept valve 18 are open so that the steam from the high pressure section can flow freely to the intermediate and low pressure sections after it has been reheated. Valves 17 and 18 take no part in the startup made according to the invention. Referring now to FIG. 2 where a hot startup is illustrated, it will be seen that within a very short time the turbine is up to synchronous speed so that loading can begin. Since the steam flow is small up to this point, there has been little heating of the turbine metal.

The procedure followed for heating the turbine from a hot startup temperature to the temperature where it can carry its rated load will now be discussed with particular reference to curves A and B of FIG. 2 illustrating the prior art. Curve A depicts the combined opening of control valves 16 and solid curve B the inside metal temperature at the first stage, i.e., inside metal temperature of the high pressure section 10, excluding valve chest 13. From the very beginning at zero time to the time indicated by the vertical line 19, the control valves remain wide open as indicated by the horizontal portion of curve A, and steam is admitted to the chest under the control of the bypass valve. During this time loading of the turbine is on the bypass valve only and in response to the increasing steam flow the temperature of the first stage rises along curve B to the point 20 on line 19. Loading on the bypass valve continues until it is wide open. Thereafter a transition must be made from full arc to partial arc control where the control valves exercise control over turbine loading. However, since the bypass valve is now wide open, it is not possible to transfer control to the control valves until they are partially closed.

In the past, the transition has been made by closing the control valves in sequence during the time interval between vertical lines I? and 21 along the steep portion curve A between these lines while no additional loading took place. As there is now increased energy extracted from the steam flow through the first v stage blades, turbine cooling takes place as represented by the line extending from point 20 on line 19 to point 22 on line 21. At point 23 of time 21 the control valves are closed to the extent where they become effective in controlling turbine loading. As indicated by the horizontal portion 24 of the steam flow curve there has been no increase in the loading of the turbine during time interval 19 to 21. Once turbine control is transferred to the control valves, heating of the turbine metal is resumed along the line extending up and to the right from point 22. During the transition period 19 to 21 there was some cooling of the turbine metal and this metal must be reheated before-the temperature is back to the level at 20 where the transition began. This cooling and reheating increases the time taken to heat the turbine to the temperature where it can be fully loaded. Full loading can begin at time 25.

In essence, this invention eliminates the cooling and reheating which formerly took place during the transition period and the resultant time delay in turbine startup. According to the invention, the transition is started before the bypass valve is fully open, i.e., at a time a little earlier than that indicated at 19. During the transition loading is continued by simultaneously opening the bypass valve further and closing the control valves. This is done at a carefully controlled rate so that there is no interruption in metal heating; heating continues uninterrupted from the beginning to the end of the transition period at the same rate as clearly indicated by the steep, straight, dashed line portion 26 of curve B illustrating the new control concept of the present invention. The turbine can now be fully loaded to an earlier time 27 without exceeding the former rate of heating. In other words, the new procedure has reduced the time to full loading by the distance between the vertical lines and 27 so that startup is more rapid. With the new procedure, the steam flow curve will change course from that illustrated after reaching time line 19; its rise will not be interrupted by a horizontal portion such as portion 24. Traditional methods of control provide several fixed loading rates for different initial metal temperature conditions and then limit the temperature difference between the inside and outside metal surfaces. This new control concept allows as high a loading rate as possible without exceeding the allowable rate of inside metal temperature change k." This rate K is determined for the selected life expenditure, and the total Amount of Metal Temperature Change" for that startup. In this case it is the 0.02 percent operating curve. Controlling to a specified heating rate will prevent the turbine metal differential temperatures from exceeding the limits and will prevent excessive metal stresses from occurring in the shell castings or the rotor.

When transfer is made to partial 'arc, the steam temperature in the valve chest rises to the temperature of the steam from the boiler causing heating of the chest. There is an appreciable time lag of the chest inner surface temperature behind the steam surface temperature, requiring the transfer to be made slowly if the inner metal surface thermocouple is used in controlling the transfer to limit the chest wall differential temperature. This is the present practice as shown in FIG. 3. To apply the new control concept, much better control of the chest differential temperature is required. It is not practicable to measure the actual steam temperature in the chest, but steam pressure can be measured in the chest and has fast response to changes in valve position. Using a computer, much better transfer control is possible with the chest steam pressure signal used as the control signal. For this control the computer calculates the enthalpy of the steam to the stop valve. This enthalpy valve will not change while the steam is throttled in the bypass valve as no heat or energy is removed from the steam. This permitted steam temperature can be calculated from the outside metal temperature and the differential temperature limit. With both the steam enthalpy and temperature known the computer can calculate from formulations of steam property relationships the allowable chest steam pressure PCR. On transfer the control valves are closed rapidly only as long as the chest pressure is below the allowable PCR reference value. This action of the control valve and chest pressure are shown in FIG. 4 The calculated PCR, Pressure Chest Reference, is shown as a dashed line, which will rise as the outside metal temperature increases. The resulting metal differential temperature does not exceed the limit although it is kept high to allow maximum heating and the shortest possible transfer time.

On rapid loading the transfer to partial arc takes place while the unit is being loaded on the bypass valve. The loading rates are varied to maintain the rate of change of inside metal temperature at the first stage and the reheat bowl. The transfer under chest pressure control provides a safe heating rate for the steam chest castings. This combined action, of transfer and loading, eliminates a transfer period during which no loading would occur and also increases the full are loading rates possible in those cases where the first stage inner metal temperatures are limiting. When the control valve reaches the transfer point before or at the same time that the bypass valve gets to its transfer point the transfer can be completed, and partial are loading can continue without delay. On partial arc the control valves open to increase the load at rates necessary to continue the same rate of heating of inside metal temperature at the first sta e and the reheat bowl of the turbine.

FIG. 5 1s a rapi startup profile for a hot start with an exact match in steam and first stage shell metal temperature and with the turbine initially at 655F. Little heating occurs in this case while the unit is being accelerated and synchronized on full arc control. Once synchronized the bypass valves open more rapidly to load the unit and increase the steam flow. This increased steam flow starts heating the inside metal at the first stage as shown by the first stage shell steam temperature line.

The main steam temperature is shown increasing and the control valve starts closing for the combined transfer and loading action. The main stop valves are opened to complete the transfer action and to start the partial are loading, shown by the rising position of control valves. Now the main steam pressure is increasing with load and the main steam temperature is held constant. This action results in a high loading rate, as shown by the steam flow, and also maintains the desired constant heating of inside metal temperature. When the steam pressure is at the rated level and control valves are wide open the unit will be delivering full load although the first stage steam and metal temperatures may be below rated temperature. Full load at rated efficiency occurs when both steam temperature and pressure are at rated values.

The faster loading within a specified life expenditure per startup is possible at the cost of the slight loss in efficiency for the short time while throttle temperature is held below rated conditions. The actual loading time required is now a function of how wide a temperature control is possible from the boiler and the boiler control. CLAIMS What is claimed as new and desired to secure by Letters Patent of the United States is:

We claim:

1. The method of rapidly loading a steam turbine to rated load, said turbine initially having all the control valves for its first stage wide open, its main stop valve closed, and its bypass valve open just enough for no load turbine operation at synchronous speed; said method comprising (1) loading the turbine on said bypass valve only at a specified rate of heating of said first stage to a bypass valve condition not yet fully open; (2) making a transition from full arc to partial are control by simultaneously closing said control valves in sequence and opening said bypass valve further at a rate controlled to maintain said specified rate of first stage heating; (3) and after the transition has been made to partial arc control, loading the turbine to rated load on said control valves at a rate which maintains said specified rate of first stage heating.

2. The method of claim 1 wherein differential metal temperatures of said first stage are used for controlling the operators for all said valves.

3. The method of claim 1 wherein the steam pressure in the valve chest is used as a representation of temperatures for controlling the operators for all said valves. 

1. The method of rapidly loading a steam turbine to rated load, said turbine initially having all the control valves for its first stage wide open, its main stop valve closed, and its bypass valve open just enough for no load turbine operation at synchronous speed; said method comprising (1) loading the turbine on said bypass valve only at a specified rate of heating of said first stage to a bypass valve condition not yet fully open; (2) making a transition from full arc to partial arc control by simultaneously closing said control valves in sequence and opening said bypass valve further at a rate controlleD to maintain said specified rate of first stage heating; (3) and after the transition has been made to partial arc control, loading the turbine to rated load on said control valves at a rate which maintains said specified rate of first stage heating.
 2. The method of claim 1 wherein differential metal temperatures of said first stage are used for controlling the operators for all said valves.
 3. The method of claim 1 wherein the steam pressure in the valve chest is used as a representation of temperatures for controlling the operators for all said valves. 